Uplinker,
It is type dependent.
The later A109s, at least from the "Power" version onwards (I don't know anything about the very early versions, they are now actually classified as a different type) have a long piano hinged cowling (and a short one) along the top of the tailboom. These are held in place by a series of Dzus "wing nut" headed fasteners so the pilot can swing the cover up and over the shaft to inspect it and it's support bearings. The bearings have tell tale "goo" lines applied across the inner and outer races to check that only the intended parts are rotating and there is no slippage of the races. Some have temperature strips applied, too. The shaft inspection is part of the Check A (Daily check). The "back end" of the gearbox is cowled but limited inspection can be carried out; the linkages are mostly external anyway, on the left side where the rotor itself sits. The tail rotor control rod goes from a hydraulic servo inside the baggage bay (again should be checked as part of the Check A), through the inside of the boom and the rear rose joint attachment point can be inspected through a small hatch adjacent to the tail cone. The underside of the gearbox can be inspected for leakage and general condition at the same place. There is a perspex window on the left side of the tail cone to enable the gearbox oil level to be seen in a round sight glass.
Other, larger helicopters are not so easily inspected.